On Tuesday, the chief of army staff said some soldiers had deserted because they were scared of Boko Haram.

The soldiers had joined the army for the wrong reason, Lt-Gen Kenneth Minimah said.

The BBC's Bashir Sa'ad Abdullahi in the capital, Abuja, says his comments give credence to the view that the military is demoralised, lacking the motivation or firepower to tackle Boko Haram.

The military is said to lack the equipment to fight the insurgents

There was international outrage after Boko Haram abducted more than 200 schoolgirls in April

This is despite the fact that Nigeria's army is the biggest in West Africa, and fought rebel groups in countries such as Sierra Leone to help end brutal civil wars, he says.

Nigeria's President Goodluck Jonathan declared a state of emergency more than a year ago in Borno, Yobe and Adamawa states, saying he would give the military extra powers to fight Boko Haram.

However, the insurgency has become more deadly since then.

New York-based Human Rights Watch says at least 2,053 civilians have been killed in an estimated 95 attacks during the first half of 2014, compared with 3,600 deaths in the first four years of the conflict.

Although Boko Haram has not confirmed that it blew up the bridge, it has been blamed for similar attacks in the past, our correspondent says.

The destruction of the bridge makes it more difficult for the army to move around and cuts off the Gamboru Ngala district, where 300 people were killed in a suspected Boko Haram raid in May, from Maiduguri, the capital of Borno state, he says.

Residents of Gamboru Ngala rely on Maiduguri for their supply of goods.

Boko Haram appears to be punishing them for setting up a vigilante group to fight it, our correspondent says.

More rallies are expected to be held to mark 100 days since the abduction of the Chibok girls

The blowing up of the bride also damages the regional economy, as goods can no longer be easily transported between Nigeria and Cameroon.

Boko Haram has been fighting since 2009 to create an Islamic state in Nigeria.

In April, it sparked international outrage by abducting more than 200 girls from their boarding school in Chibok town, also in Borno state.

Rallies are due to be held in several Nigerian cities on Wednesday to mark the 100 days that have passed since their abduction.